1. Field of the Invention
This invention relates to a method of sterilizing foods using a combination of ultra-high pressure and elevated temperatures. More particularly, the invention relates to using the synergy between the adiabatic temperature rise that occurs when a material is hydrostatically pressurized, coupled with the lethality of the pressure to achieve the appropriate sterilization conditions.
2. Description of the Related Art
The potential to process certain foods using ultra-high pressure (UHP) sterilization has been known since the turn of the century when hydrostatic pressures in excess of 100,000 psi at ambient temperatures were tested and it was established to have a lethal effect on vegetative bacteria. The process involves pressurizing a material (in this case a food) to ultra-high pressures (50,000 to 150,000 psi and higher). This process is very effective at eliminating vegetative bacteria, yeast and molds. The treatment is uniform throughout the product and much faster in its ability to inactivate microorganisms than standard batch sterilization processes which heat the food product slowly through conduction. UHP is often referred to as a "heatless" or "cold pasteurization" method. In the literature, it was believed that UHP was not very effective at destroying bacterial spores or denaturing enzymes. These are the primary requirements for processing low acid, shelf stable, canned foods.
The recent increase in consumer interest in higher quality foods has caused the food industry to become interested in UHP since standard practice provides only for low temperature pasteurization of low acid products and commercial sterilization of high acid products. The advantage of UHP over typical thermal processes is the potential to increase shelf life without significantly degrading the nutritional characteristics, flavor and color quality of the food. Chemical reactions/degradation that occur as a result of thermal treatment are virtually eliminated and the process is potentially more economical from an energy utilization perspective.
The Japanese were the first to commercialize UHP in 1990 when MEIDI-YA introduced a UHP preserved jam into the retail market. Currently, several high acid UHP processed products can be found in the Japanese market, including fruit, yogurt, jams, jellies and fruit sauces.
Ultra-high pressure bacterial inactivation is not well understood. It is believed that microbes are destroyed through altered permeability of the cell membranes from mechanical disruption as well as protein denaturation due to the disruption of hydrophobic bonds, ionic bonds and subsequent unfolding of the protein structure. In contrast, thermal protein denaturation and to a large extent microbiological inactivation is due to the destruction and creation of covalent bonds. It is currently believed that the UHP process is only effective for inactivating vegetative bacteria, yeast and molds.
Therefore commercial treatments are limited to high acid food sterilization or low acid food pasteurization. Low acid food pasteurization involves heating the product to 60-100.degree. C. and is only effective for inactivating non-spore forming pathogens. The sterilization process is particularly severe because of the time required to heat the entire product, particularly the center of the product, to the treatment temperature (&gt;100.degree. C.). That is, by the time the center core of the product achieves the desired peak treatment temperature for the desired period of time, the outer portions of the product have been overtreated. Accordingly, low acid food sterilization, particularly in packaging (which tends to insulate the product), is undesirable since the extended time for thermal treatment often degrades the characteristics of the product.
The following references and those referred hereinafter, each of which are hereby incorporated by reference, disclose the state of the art.
Japanese Patent Publication 225 7864 to Ajinomoto discloses pressure sterilization of bacterial spores. The publication discloses sterilizing bacterial spores under pressure by treating a food product for 5 to 300 minutes at 30 to 100.degree. C. under a pressure of 1,000 to 10,000 kg/cm.sup.2 (70-700 psi) (not ultra-high pressure).
Japanese Patent Publication 318 3450 to Dainippon Printing discloses the preparation of cut vegetables involving the step of pasteurizing the product by applying a pressure of at least 1,000 kg/cm.sup.2 (70 psi) (not ultra-high pressure).
Australian Patent Publication 42 50 72 to Donald discloses the sterilization of food compositions. The disclosed process includes raising the pressure of a previously heated food product, injecting steam into the pressurized chamber to allow the steam to condense onto the product, raising the temperature of the composition and subsequently releasing the pressure. The publication discloses that the composition is held at a pressure such that the steam is condensed to water donating its latent heat energy to the surrounding composition and subsequently lowering the pressure, causing the condensed water to flash off, taking its latent heat energy from the composition and thus cooling it.
Although high pressure sterilization has been used to treat high acid foods in the past, the prior art does not disclose the ultra-high pressure sterilization of low acid foods. It would be desirable to develop a method of processing foods to commercial sterility without subjecting the food to thermal degradation.